1. Field of the Invention
The present invention relates to a golf ball and more particularly to a dimple pattern of the golf ball.
2. Description of the Related Art
A golf ball has approximately 300 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight of the golf ball to accelerate transition of a turbulent flow at a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backwards so that a pressure resistance is reduced, resulting in an increase in a flight distance of the golf ball. Moreover, the acceleration of the transition of the turbulent flow increases a differential between upper and lower separating points of the golf ball which is caused by back spin. Consequently, lift acting on the golf ball is increased. Accordingly, a dimple pattern capable of easily accelerating the transition of the turbulent flow, that is, a dimple pattern capable of better disturbing an air stream is more excellent aerodynamically.
A regular polyhedron or a quasi-regular polyhedron (which will be hereinafter referred to as a xe2x80x9cpolyhedron) is often used for the dimple pattern. More specifically, a polyhedron inscribed on a sphere is supposed, and sides of the polyhedron are projected on a spherical surface by rays irradiated from the center of the sphere onto the spherical surface, thereby forming an edge line. The spherical surface is comparted by the edge line. Thus, the dimples are arranged. Examples of the polyhedron to be used include a regular hexahedron, a regular octahedron, a regular dodecahedron, a regular icosahedron, a cube-octahedron, an icosa-dodecahedron and the like.
The regular octahedron has been admired for a general golf ball for a long time because dimples are systematically aligned finely. The edge line obtained by projecting the sides of the regular octahedron forms three comparting lines having the shape of a great circle. These comparting lines are orthogonal to each other. The spherical surface is comparted into eight spherical regular triangles through the comparting lines. Dimples are arranged over each of the spherical regular triangles completely equivalently or almost equivalently. Such a dimple arrangement is referred to as a regular octahedron pattern.
The golf ball is formed by a mold comprising upper and lower half-molds including semispherical cavities. A spew is generated in a portion corresponding to a parting line of the upper and lower half-molds on the surface of the formed golf ball. The spew is ground and removed through a grindstone or the like. For easy removal, dimples are not usually formed on the parting line. In an ordinary regular octahedron pattern, dimples are not present on the three comparting lines. One of the comparting lines is coincident with the parting line. Such a golf ball has been disclosed in Japanese Laid-Open Patent Publication No. 60-11665 (1985).
In the golf ball having the regular octahedron pattern, dimples are not present on the parting line. Therefore, a dimple effect tends to be insufficient when the parting line (to be the comparting line) is coincident with a portion in which a circumferential speed of back spin is the highest (which will be hereinafter referred to as the xe2x80x9chighest speed portionxe2x80x9d). As described above, the spew generated on the parting line is removed by the grinding. Therefore, there is a possibility that the vicinity of the parting line on the surface of the golf ball might be ground simultaneously during the removal and the dimples might be deformed, resulting in a reduction in the dimple effect. Furthermore, the dimple patterns on the right and left of the parting line are equivalent to each other and the equivalent dimple patterns are repeated along the parting line. Therefore, the dimple effect tends to be insufficient when the parting line is coincident with the highest speed portion. More specifically, in the golf ball having the regular octahedron pattern, the following three unfavorable conditions are satisfied on the parting line:
(1) the parting line is a great circle path having no dimple;
(2) dimples provided around the parting line might be deformed by grinding; and
(3) a dimple pattern appearing along the parting line by rotation is monotonous.
Japanese Laid-Open Patent Publication No. 11-70186 (1999) has disclosed a golf ball having a regular octahedron pattern in which dimples are present on a comparting line. In the golf ball, the dimple effect can be enhanced by the dimples present on the comparting line when a parting line or the vicinity thereof is coincident with the highest speed portion. In other words, the unfavorable condition of (1) can be eliminated. However, the unfavorable conditions of (2) and (3) are still made on one of the comparting lines.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a golf ball making the most of advantages of a regular octahedron pattern which is systematically aligned finely and having no great circle to considerably reduce a dimple effect in coincidence with the highest speed portion.
In order to achieve the above-mentioned object, the present invention provides a golf ball in which dimples are arranged equivalently or almost equivalently over eight spherical regular triangles obtained through three great circle-shaped comparting lines formed by projecting sides of an inscribed regular octahedron and any great circle path to be a great circle which does not intersect the dimples is not present,
wherein any of the three comparting lines is not coincident with an equator line.
In the golf ball, the equator line is not coincident with the comparting line. Therefore, a portion in which the dimples might be deformed due to grinding (the vicinity of the equator line) is not coincident with a portion in which a dimple pattern appearing by rotation is simple (the vicinity of the comparting line). Accordingly, there is not a great circle which simultaneously satisfies unfavorable conditions of (2) and (3), that is, a great circle which reduces the dimple effect extremely in coincidence with the highest speed portion. In addition, the golf ball does not have a great circle path (which does not intersect the dimples). Consequently, the golf ball is excellent in a flight distance and aerodynamic symmetry. Moreover, the golf ball has the dimples arranged equivalently or almost equivalently on the eight spherical regular triangles. Therefore, the dimples are arranged finely and have a good appearance.
The equator line implies a line corresponding to a of latitude line having an zero degree latitude on the assumption that a golf ball formed by upper and lower molds is a terrestrial globe, the uppermost portion of an upper cavity is the North Pole and the lowermost portion of a lower cavity is the South Pole. In the case in which the upper and lower molds include a completely semispherical cavity, the equator line is coincident with a parting line.
As described above, the great circle path is not present in the golf ball. Therefore, the equator line also intersects the dimples. Preferably, any dimple intersecting the equator line does not centrally intersect the equator line. The central intersection implies the intersection of the dimple with a line such that the line passes through the center of the dimple (a center of gravity of a non-circular dimple). Since the dimple intersecting the equator line does not centrally intersect the equator line, an intersecting dimension of the dimple and the equator line (a distance between a line parallel with the equator line and close thereto in contact with the dimple and the equator line) is less than xc2xd of a dimple diameter. The golf ball having no great circle path is usually formed by a mold in which a parting line has a concavo-convex shape. If the intersecting dimension is less than xc2xd of the dimple diameter, the parting line does not need to be considerably kept away from the equator line. The extent of concavo-convex portions can be reduced. Accordingly, it is not very hard to fabricate the mold and to form the golf ball.
Preferably, the number of the dimples intersecting the equator line is 12 to 24. Consequently, the prevention of a reduction in the dimple effect obtained when the equator line is coincident with the highest speed portion is compatible with easiness of manufacture of the golf ball. Moreover, it is preferable that the intersecting dimension of all the dimples intersecting the equator line is less than xc2xc of the dimple diameter. Consequently, the extent of concavo-convex portions of the parting line can be reduced still more.
Preferably, the three comparting lines centrally intersect 16 dimples or more, respectively. Consequently, it is possible to prevent the dimple effect from being reduced when the comparting line is coincident with the highest speed portion.
Preferably, the equator line is coincident with a great circle obtained by connecting middle points of sides of the spherical regular triangle. Consequently, when the golf ball is rotated once along the equator line, three spherical isosceles triangles and three spherical trapeziums appear alternately on right and left sides of the equator line, respectively. Accordingly, the dimple pattern appearing by the rotation is not monotonous and the dimple effect can be prevented from being reduced when the equator line is coincident with the highest speed portion.
The present invention will be described below in detail based on an embodiment with reference to the drawings.